<div>
    Specifies the Groovy filter expression that restricts the subset of the combinations
    that the downstream project will run.

    <p>
    See the "Combination Filter" field in a matrix project configuration page for more
    details about the environment the script runs in, examples, and so on.

    <p>
    What you specify here gets expanded by variables of the triggering build, which allows
    you to dynamically control the subset of the triggered job. For example, if you trigger job BAR
    from FOO with <tt>label=="${TARGET}"</tt> in the filter, and if FOO defines the <tt>TARGET</tt> variable
    and FOO #1 sets <tt>TARGET</tt> to be <tt>linux</tt>, then the triggered BAR #3 will only select the subset
    of combinations where <tt>label=="linux"</tt> holds true.

    <p>
    Note that the variable expansion follows the <tt>${varname}</tt> syntax used throughout in Jenkins, which
    collides with Groovy string inline expression syntax. However, Jenkins variable expansion leaves undefined
    variables as-is, so most of the time your Groovy string line expression syntax will survive the expansion,
    get passed to Groovy as-is, and work as expected. If you do need to escape '$', use '$$'.
</div>
